The present invention is directed to fabric filter media for commercial and industrial applications, and more particularly, to a stiffened, pleated filter medium for high-temperature gaseous filtration applications.
Over the last several decades, there has been an increased awareness and emphasis on environmental issues, including air and water quality. Increasingly strenuous governmental controls and limits have been imposed to preserve these natural resources and prevent associated health hazards.
Numerous advances have been made in effectively controlling air pollution by removing undesirable particular matter from gaseous processing streams and exhausts. One particular type of air filtration technique has been the employment of particulate collectors known as xe2x80x9cbaghousesxe2x80x9d. Baghouses operate much like a household vacuum cleaner. Dirty air is drawn into a chamber, or plenum, where a filter medium is positioned. The air passes through the filter into a clean air plenum, depositing the particulate matter, dust, etc. on the filter surfaces as it passes into the clean air plenum. Because of the high volumes of air being processed in industrial applications, these baghouses must be equently cleaned. Cleaning is typically conducted by reversing the flow of air or gas through from the clean air plenum into the dirty air plenum, dislodging the dust and particulate matter that has accumulated on the surfaces of the filter. In some industrial applications, these cleaning cycles must be repeated hundreds of times each day. As will be appreciated, the filters very quickly become structurally fatigued, tearing in short periods, thus requiring frequent, costly replacement.
The cleaning problems and failures of baghouse filters, including cartridge filters, are further exacerbated when the filters are installed in high temperature applications where the gaseous discharges exceed 450 degrees Fahrenheit. Fabric filter media very quickly deform and deteriorate, rapidly losing their structural rigidity and filtration efficiency. As a result, filtering high temperature gaseous discharges to meet regulatory requirements in a cost-effective manner, has been problematic.
The present invention is directed to a fabric filter material and a pleated filter formed therefrom that address the problems of filtering gaseous streams in high temperature applications; i.e., at temperatures between about 450 and 550 degrees Fahrenheit.
A preferred embodiment of the present invention provides a fabric material to which a stiffener is applied for maintaining the form (rigidity) of the fabric in the spectrum of high temperature applications. A fibrous fabric material that is capable of being stiffened and formed is selected. In this embodiment, the fibrous fabric is woven entirely from yarns of fiberglass, preferably type ECDE (continuous filament electrical grade) yarns in which a portion of both the fill and warp yarns are air-jet texturized. It has been found that using texturized yarns substantially increases the wet pickup of the fabric and facilitates the more precise forming (shaping) of the fabric. xe2x80x9cWet pickupxe2x80x9d refers to the amount of a liquid finish, expressed as a percentage, that a finished fabric will absorb. The greater the wet pickup, the more effective the stiffener is in maintaining the form of the woven fabric during its anticipated high temperature service. It has also been found that fiberglass, as a filter material, retains superior durability after frequent and numerous fatigue cycles, and is quite adaptable to stiffening and forming.
The preferred stiffener is comprised of a resorcinol-formaldehyde resin solution, an acrylic resin emulsion, ammonia, hexamethylenetetramine, and water. The stiffener may be applied using any of the conventional methods known in the art for applying finishiners such as dipping, spraying, etc. Once the stiffener is applied, the treated fabric is heated until dry, but at a temperature and duration that will not exceed B-stage curing of the treated fabric. This allows the treated fabric to be later formed and set in a desired shape.
In a preferred embodiment, the stiffening system of the present invention includes three discrete layers that are sequentially applied to the woven fiberglass fabric, that is, two additional layers that complement the stiffener. An initial, or inner, layer serves as a lubricant for the subsequently applied stiffening layer and consists of water, a lubricant (desirably a silicon such as a phenyl silicone polymer because of its high temperature stability), and a polytetrafluorethylene (PTFE) dispersion. After the lubrication layer is applied, it is heated within a specified temperature range for a specified duration until dry. The stiffening layer is next applied. This layer is comprised of a resorcinol-formaldehyde resin solution, an acrylic resin emulsion, ammonia, hexamethylenetetramine, and water. The treated fabric is again heated until dry, but at a temperature and duration that will not exceed B-stage curing of the treated fabric. This allows the treated fabric to be later formed and set in a desired shape. Lastly, a protective layer is applied to the treated fabric, the protective layer consisting of a PTFE dispersion and water. The treated fabric is heated until dry, but again at a limited temperature and duration combination so that the fabric will not cure beyond the B-stage. The finished treated fabric may be immediately shaped and set, or may be stored in the B-stage for later forming.
In a preferred embodiment, the treated fabric is pleated as it is well known in the filtration arts that pleated filters provide substantially more (2 to 3 tires) filtration surface area for a selected filter size. Any of the known commercial pleating machines may be used to pleat the fabric. Sufficient heat and duration are required to set the pleats, fully curing and setting the treated fabric. An oven or infrared lights downstream of the pleating operation are employed to provide this curing, setting heat.
A further embodiment of the present invention is a filter device, such as a pleated cartridge filter, for high temperature filtration applications. Such a cartridge filter is easily adapted to the baghouse filter systems described above. The filter device is comprised of a perforated liner, a generally circular pleated portion of treated, stiffened fabric, at least one retainer, and end flanges. The perforated liner is preferably a metallic cylinder with open ends and perforations formed through and spaced about the cylinder walls. The perforations are sized and spaced to optimize the flow of air into the clean air plenum after passing through the pleated filter. The most important function served by the liner, however, is structural support for the surrounding fabric filter. Completely surrounding the outer wall surface of the cylindrical liner is the pleated fabric filter material. At least one retainer, such as a band, strap, wire, etc. holds the filter material in place around the cylindrical liner. This especially provides additional rigidity and support to the filter material during the frequent cleaning cycles which force air in a reverse flow through the liner and back across the filter material to dislodge trapped particulate. Finally, to secure the upper and lower free ends, or edges, of the pleated filter, caps or flanges are fitted around the ends or edges and adhered to the fabric with a conventional potting compound known in the art and adapted for high temperature applications.
The fabric filter material treated and formed as described hereinabove and in accordance with the detailed description that follows, is capable of achieving a filtration efficiency of greater than 99% for particulate matter of 10 microns or greater. Further, when pleated and incorporated into a filter device, such as a filter cartridge, the fabric filter material can withstand thousands of cleaning cycles without failure.